Hard disk drive incorporating rotating magnetic disks is commonly used for storing data in the magnetic media formed on the disk surfaces, and a movable slider including reader sensors are generally used to read data from tracks on the disk surfaces.
FIG. 1a is a simplified perspective view of a conventional reader sensor 600, and FIG. 1b is a schematic view of the reader sensor 600 seen from air bearing surface (ABS) (not shown) of a slider. As shown in FIGS. 1a and 1b, the reader sensor 600 includes a first shielding layer 601, a second shielding layer 602, a read element 650 formed therebetween, and a pair of permanent magnet layers 603 respectively placed on two sides of the read element 650.
Commonly, the permanent magnet layers 603 provides a longitudinal bias field to the read element 650 for stabilizing the read element 650 to some extent, which has a magnetic direction 11 parallel to the ABS. An outer signal field coming from a magnetic medium, such as a disk of a disk drive unit will be supplied to the read element 650, which has a magnetic direction 10 perpendicular to the ABS. As known, a free layer of the read element 650 having the shape anisotropy effect will rotate as a result of the outer magnetic field, thus the magnetization direction of the free layer will shift freely. Thus the longitudinal bias field with a fixed direction can stabilize the free layer to a degree.
However, this reader sensor still may be unstable for the changing external environment or the other internal elements, which will impact the reading performance of the magnetic head. If excessive and obvious noise and jumping appear during the stability testing, the magnetic head having such reader sensor will be rejected. Thus, the defective magnetic head will be abandoned as a defective product, which causes a waste and an increased manufacturing cost.
Hence, it is desired to provide an improved reader sensor to overcome the above-mentioned drawbacks.